This invention relates generally to injection molding and more particularly to a method of manufacturing an injection molding nozzle having an integral heating element in which both brazing and casting are carried out in a single heating cycle of a vacuum furnace.
Making an injection molding nozzle with an integral electrical heating element has many advantages such as improved heat transfer, reduced corrosion and longer operating life. It is well known to make such integral nozzles by first sealing the components together to form a space around a helical portion of an electrical heating element, usually by brazing in a vacuum furnace. The nozzle is then reinserted into the vacuum furnace to cast a conductive material such as copper into the sealed space around the helical portion of the heating element. In the previous methods, a conductive material such as a beryllium copper alloy was selected to have a lower melting temperature than the brazing material. Different variations of this method are described in the applicant's U.S. Pat. Nos. 4,355,460 which issued Oct. 26, 1982, 4,403,405 which issued Sep. 13, 1983, and 4,771,164 which issued Sep. 13, 1988. While these previous methods have many advantages, they all have the disadvantage that the sealing of the space around the helical portion of the heating element and the casting of the copper into this space are two separate steps requiring two different cycles of the vacuum furnace. Furthermore, in the past, integrally casting the heating element in the conductive material has improved the extremely critical factor of thermal conductivity by reducing air pockets in the nozzle around the heating element. It has now been found that thermal conductivity can be further improved by cooling the nozzle in a manner to provide unidirectional solidification of the conductive material from the bottom up to prevent the formation of voids due to shrinkage.